1. Field of the Invention
The present invention relates to contact lenses for reshaping corneas, and more particularly relates to contact lenses for reshaping altered corneas which are geometrically abnormal, such as post-refractive surgical corneas for the enhancement of residual myopia, for eliminating over-treated hyperopia and/or presbyopia, and for reshaping irregular corneas such as advanced or severe Keratoconus to restore usable spectacle vision. More particularly, the invention relates to contact lenses that are shaped to provide gradual altering of the patient's cornea during continued wear to reshape the cornea to eliminate the residual myopia, hyperopia, presbyopia, or astigmatism of altered cornea condition. The lens may also be adapted for enhancement of previously Ortho-K altered corneas for adding on myopia reduction.
2. Art Background
Many people experience difficulties with their vision due to a number of possible conditions. The most common vision problem is a condition known as myopia or nearsightedness. Myopia is a common condition where an eye cannot focus on far-away objects because the cornea of the eye is curved too steeply (i.e., where the radius of curvature of the cornea is smaller than normal) to provide adequate focusing at the retina of the eye.
Another condition is known as hyperopia or farsightedness. With hyperopia, the eye cannot focus on both far and near objects because the curvature of the cornea of the eye is too flat (i.e., where the radius of curvature of the cornea is larger than normal) to provide adequate focusing at the retina of the eye. Hyperopia is common among young children. Severe hyperopia will induce lazy eye or amblyopia in childhood. Mild or moderate hyperopia is tolerable and insidious in young ages but will cause reading problems in older age.
Yet, another common problem is astigmatism, where unequal curvature of one or more refractive surfaces of the cornea prevents light rays from focusing clearly at one point on the retina, resulting in blurred vision. Presbyopia is the most common vision problem in adults 40 years and older. It does not matter whether they are emmetropic (normal condition), myopic or hyperopic in far vision, the middle-aged population, i.e. over 40 years old, will begin to experience difficulty in focusing on close objects, due to the loss of flexibility of the eye's crystalline lens. Presbyopia may occur and complicate other refractive problems such as hyperopia, myopia or astigmatism.
A normal cornea is usually parabolic in shape, which is steepest (shorter radius) in curvature at, or nearly at, the central portion of the cornea and becomes progressively flatter (longer radius) in curvature to the limbus by certain positive e-value, or so-called “positive shape factor”. An altered cornea is a cornea that is far different from the normal parabolic shape, having an abruptly protruded portion or a “negative shape factor” of a human cornea, which may occur naturally, or results from some refractive surgical procedures.
The former condition, i.e. “naturally altered,” is best demonstrated by Keratoconus, which is manifested by an abruptly protruded cone that is usually located slightly inferior (i.e. lower portion of) to the cornea. It would be difficult to obtain useful vision in advanced or severe Keratoconus by spectacles or any type of contact lenses. The final choice to restore useful vision will be cornea transplantation traditionally. However, complications accompanying cornea transplantation are very common such as irregular astigmatism, graft rejection, infection or recurrence of Keratoconus. That's the reason why any non-surgical method that may rehabilitate Keratoconus and avoid cornea transplantation will be quite valuable.
The latter condition, i.e. due to surgical procedures, is best illustrated by the myopic refractive surgery such as LASIK, PRK and RK. The post-operative cornea is usually manifested by an ablated, flattened curvature at the center portion of the cornea. It is not uncommon to still have unsatisfactory vision, even after the refractive surgery, such as residual myopia, over-corrective hyperopia, iatrogenic Keratoconus or irregular astigmatism. The usual ways to manage the post-operative complications are enhancement operations, wearing glasses, fitting contact lenses or cornea transplantation for the severe cases.
Another conventional approach to treating some or all of these refractive errors is to alter the corneal shape by wearing contact lenses which are designed to continually exert pressure on selected locations of the cornea to gradually force or mold the cornea into the desired corneal curvature. A retainer lens is then worn on a part-time basis to prevent the cornea from returning to its previously deformed shape. This method of treatment is commonly referred to as orthokeratology (referred to hereinafter as “Ortho-K”). While Ortho-K is traditionally applied to the normal corneas to correct myopia, astigmatism and hyperopia, its application to the altered corneas has been unexplored, since it is considered very difficult to figure out a proper lens for reshaping the altered corneas. The cornea curvatures of these altered corneas are quite irregular, not measurable, or flattened artificially at the central portion of the cornea. It would be difficult to measure or to apply the conventional cornea information, such as cornea curvatures or eccentricity value of the altered cornea, for preliminary cornea reconstruction to custom make the Ortho-K lenses.
For example, conventional Ortho-K contact lenses with a longer central radius of curvature than the central radius of the cornea are known to change the shape of the cornea by compressing the surface at its apex. This reshaped cornea has a lengthened radius of curvature in its central zone, which serves to improve myopia. However, on an altered cornea of post refractive surgery, it would be very difficult to figure out a lens that can compress the dimpled central portion of the altered cornea for increasing or furthering myopia reduction. It is especially true if the original cornea has already been ablated a lot to correct high myopia as that of higher than −8 or −10 diopters. The higher the original myopia the more cornea tissue will be removed during the refractive surgery, and hence more likely to have post-operative vision problems.
Ortho-K has been performed in one form or another since the early 1970s. Almost all the lenses are designed to mold normal, regular corneas that are parabolic in shape. Modern Ortho-K lenses are usually designed to precisely match the cornea surface by obtaining information from cornea measurement. The measured information, such as cornea curvatures and eccentricity value, is then put into mathematical calculation, known as “preliminary cornea reconstruction,” which in turn forms the basis for figuring out lens specifications for manufacturing. It was thought very difficult to figure out the precise lens specifications for molding the altered corneas that are flattest at center portion of the cornea (post-refractive surgery), or abruptly protruded cornea (Keratoconus). Some practitioners have applied the conventional Ortho-K lenses, “trial-and-error,” method (piece-by-piece) trying to mold the altered corneas, but are rarely successful. It would be worthwhile to provide a non-surgical method to mold the altered corneas by Ortho-K, which will save the requirement of enhancement operation or corneal transplantation.
The upper limit of Orthokeratology has been thought to be −6.00 diopters. Although it is possible to reduce myopia up to −10.00 diopters by the lens design disclosed in my U.S. Pat. No. 6,543,897, it is still useful to figure out a contact lens that will further mold the altered cornea by previous Ortho-K to achieve further Ortho-K reduction in excessive high myopia. The difficulty of furthering reduction on the altered cornea of previous Ortho-K is quite similar to that of enhancing the altered cornea of post refractive surgery for adding on myopia reduction.
U.S. Pat. No. 5,963,297 to Reim and U.S. Pat. Nos. 5,349,395, 4,952,045, 5,191,365, 6,010,219 to Stoyan disclose Ortho-k lens designs for myopia reduction. There has been no disclosure of lenses specifically designed for reshaping the altered corneas. Notwithstanding the improvements provided by modern Orthokeratology for myopia, there remains a need for a contact lens that can be used for effective Ortho-K treatment of altered corneas to restore useful vision by a non-surgical way, thus avoiding the enhance operation or cornea transplantation.